In addition, the first bbq book I ever had was Paul Kirk's Championship BBQ, that started me on the mustard slathers from day one, way before doing this professionally, and I've used it ever since. I think just mustard is too thick for a slather, and it's about binding the rub to the meat, so mixing in some leftover pickle juice or anything to thin it out a bit is great. The slather, I've found, doesn't do anything for the flavor, but does create a firm yet supple bark on the meat. I've cooked on many different types of pits(a lot of them self made), but have come to the conclusion that the slathers only work on an offset pit with a strong draft flowing over the meat, and cooking fat side up. The bark that can be created on the rendered fat is just mind blowing!!! It's totally different, than bark created with sugar containing rubs on the bare lean side of brisket, which is standard in KCBS competition. Side note here, get "Legends of Texas BBQ" by Robb Walsh. I think this one and Paul Kirk's combined are the perfect combination!
As far as just salt and pepper. I've found it to be boring. Keep that rub really granular though, too many spice powders in the mix make for a pasty and soggy bark (especially if you use foil). Try out the salt and pepper rub, but replace half of the salt portion with seasoned salt. I first got the idea when visiting the number one bbq joint in Texas when the last top 50 came out. I detected it in the flavor and saw it sitting around there. I went back home and tried it out and loved it.
Hope some of this helps out, and please don't start injecting pickle juice into briskets!!!